The rod-in-tube technique is well-known in the art. In this technique a rod of a core material is usually placed in a tube of cladding material and the assembly, optionally after the tube has first been contracted on the rod by a thermal treatment, is drawn to form an optical fibre.
It was established that in a modified embodiment of this method in which the rod consists of a core material and a cladding of quartz glass the optical fibre obtained by means of the rod-in-tube technique sometimes did not satisfy certain minimum requirements as regards the tensile strength.
It was found that fracture occurring upon determining the minimum tensile strength (screen test) in such a case usually is the result of contaminations which are present at the outer surface or in a zone of low depth below the outer surface of the quartz glass tube in which the rod is placed.
The said contaminations may be oxides having a higher melting point than quartz glass, such as aluminum oxide, chromium oxide and zirconium dioxide. In commercially available quartz glass tubes manufactured from quartz crystals, both alien particles and conglomerates of alien particles have been found to occur usually immediately below the outer surface of the tube.
These particles probably originate from the manufacturing process. The dimensions of the said conglomerates in the axial and tangential directions of the tube are at most 100 .mu.m. They are present directly below the outer surface in a zone having a thickness of 10 .mu.m.
Etching such quartz glass tubes with the object of removing the contaminated surface zone is possible, for example, by means of a hydrofluoric acid solution. However, small pits are formed in the surface while it is found in practice that the contaminations which adversely influence the tensile strength are difficult to remove entirely in this manner.
The invention is based on the recognition that contaminations in the form of oxidic particles can be dissolved to a sufficient extent by means of a thermal treatment to eliminate the adverse effect of said particles on the mechanical properties of the finished optical fibre.
The method according to the invention is characterized in that the following steps of the method are carried out in the following sequence:
a layer of core glass is provided on the inside of a tube of quartz glass; PA1 the internally coated tube is contracted to form a solid rod of the same or substantially the same length; PA1 the outer surface of a tube of quartz glass is heated at a temperature of 2100.degree. C. or higher for a period of time which is sufficient to dissolve contaminations consisting of high-melting-point oxides which are present in a surface layer at a depth of 10 .mu.m to a sufficient extent to eliminate the adverse effect on the mechanical properties of the finished optical fibre; PA1 the solid rod is placed in the thermally pretreated tube of quartz glass; PA1 after which the assembly of rod and thermally pretreated tube is drawn to form an optical fibre.
In this method the detrimental particles disappear entirely or their dimensions become so small that the critical fracture limit is no longer exceeded by the dimension of the particles, the critical fracture limit being that set forth in Griffith's law which says that the critical fracture limit is proportional to the root from the reciprocal length of a discontinuity.
In the method according to the invention the known internal deposition methods for the manufacture of optical fibres may be used, for example, MCVD (modified chemical vapour deposition) and PCVD (plasma activated chemical vapour deposition). See, for example, the article by Dr. G. Koel: "Technical and Economic Aspects of the Different Fibre Fabrication Processes", in Proc. 8th European Conf. on Optical Communication (8 ECOC), Cannes, Sep. 1982, p. 108.
The contracting of the internally coated tube takes place in the usual manner by means of a thermal treatment in which the tube is contracted to form a rod under the influence of the surface stress and pressure difference. During the contraction the tube is rotated. The length of the rod is equal or substantially equal to that of the starting tube. The resulting rod is placed in the quartz glass tube. The inside diameter of the tube need only be so much larger than the diameter of the rod that the latter can easily be inserted into the tube. Before the rod is placed in the quartz glass tube, the latter is subjected to a thermal treatment. It is sufficient when the tube is heated to a temperature of 2100.degree. C. or higher down to a depth of approximately 10 .mu.m.
Suitable means to perform such a heating are thermal sources which can transfer a large amount of thermal energy in a short period of time, for example, a hydrogen-oxygen burner or a plasma burner.
A plasma burner is preferably used in which the plasma is formed in a gas which consists at least partly of a molecular gas, for example nitrogen and oxygen. The energy transfer of such a plasma to the surface to be heated takes place substantially by recombination of dissociated molecules at the surface. The binding energy of the molecules of the gas is released. This leads to very high temperatures in a thin surface layer so that conglomerates of alien particles can be dissolved in the quartz glass within a few seconds.
It has been found advantageous in practice to subject the outer surface of the tube of quartz glass to a wet-chemical etching treatment before the outer surface is heated to a temperature of .gtoreq.2100.degree. C. Etching may be carried out, for example, by means of an aqueous hydrofluoric acid solution, for example, a solution of 15-20% by weight of HF in water. Upon etching with hydrofluoric acid the conglomerates of alien particles are not attacked or hardly attacked but the surrounding quartz glass is attacked. After reaching a certain etching depth (approximately 10 .mu.m) parts of conglomerates work loose from the surface already with very low forces; such forces occur, for example, upon rinsing away the etchant. The resulting cavities in the surface are closed within a few seconds in the subsequent thermal treatment at a temperature of .gtoreq.2100.degree. C., alien particles, if any, still present in the cavities dissolving. Preferably, during the thermal treatment, the inside of the tube of quartz glass is brought and maintained at a pressure exceeding the ambient pressure. The overpressure is, for example, approximately 50 Pa. As a result of this, variations of the inner and outer diameters of the tube in an uncontrollable manner by contraction of the tube under the influence of surface stress. In this manner, however, it is also possible, by a suitable choice of the overpressure and thermal treatment, to bring the tube at a diameter smaller than the starting diameter in a controllable manner and hence to obtain a better fitting between rod and outer tube.
In a modified embodiment of the method according to the invention the resulting rod is placed in a quartz glass tube after which the quartz glass tube is contracted on the rod to form a solid assembly. The outer surface of the solid assembly is then heated at a temperature of 2100.degree. C. or higher for a period of time which is effective to sufficiently dissolve contaminations consisting of high-melting-point oxides which are present in a surface layer at a depth of 10 .mu.m to eliminate the adverse effect on the mechanical properties of the finished optical fibre. In a subsequent step the thermally treated solid assembly is drawn to form an optical fibre.
In this modified embodiment of the method in accordance with the invention it is also recommendable to subject the outer surface of the quartz glass tube to a wet-chemical etching treatment. Said treatment is preferably carried out before the tube is united with the rod to form a solid assembly.